Photodynamic therapy, PDT, is presently undergoing extensive basic, preclinical and clinical testing for the diagnosis and treatment of cancer. Regulatory approval for the first PDT drug, Photofrin-II is expected in late 1991. Concurrent with this is the introduction of a number of second generation PDT drugs into clinical trials. all of these clinical trials are based on the use of laser light sources for the activation of the photosensitizing drug. A laser light source, though, is not required for those non-endoscopic applications using surface illumination. It is the aim of this phase-I project to develop and test a prototype light emitting diode (LED) source for PDT of external surface lesion using second generation photosensitizers. The LED offers a potential cost effective, efficient and effective light source for a number of PDT applications. Its extremely small size permits the use of multiple LED, or arrays, to provide a sufficient irradiance to activate the PDT therapeutic reaction in-vivo. The prototype system will include simultaneous delivered space irradiance measurement of the delivered light, along with thermal monitoring at the tissue/LED interface. A prototype system will be tested in a rat tumor system using the PDT drug, SnET2. In phase-II a clinical prototype system will be developed and tested in humans with the SnET2 or other new PDT drugs. If the system can be developed into a easy to use clinical system, its application in PDT would be significant, especially with the new photosensitizers that have absorption at longer wavelengths and exhibit minimal skin photosensitivity problems. The commercial potential of such a system would be great since it would lower cost and therefore make the PDT procedure a potential office type treatment.